How do I mirror a object without the mirror modifier?
This works both in Object and in Edit mode.


Place the 3D cursor where the mirror axis origin would be (Shit+C will place it at position (0, 0, 0)
Change Pivot center for rotation or scaling to be 3D cursor (or press . (dot) on the keyboard)
Duplicate the object or part of mesh you want to mirror (Shift+D)
Press Ctrl+M (or find command Object->Mirror Mesh->Mirror) and then select the axis you want to mirror the duplicated object along (in interactive mode press X, Y or Z on the keyboard). Click left mouse button to confirm.


In order mirroring to work correctly in Object Mode you have to Apply Scale and Rotation on the object first.